Tim
I am not sure why keepalives would be off on a single interface other than the possibility that some one at some time turned it off.
In general I believe that best practice is to have keepalive enabled on HDLC interfaces.
I believe that the primary advantage is that the router proactively determines whether the interface/link is working and therefore whether it is safe to send data over the link. The way that HDLC keepalives work is that each end of the link sends keepalive packets to the other end. Included in the keepalive packet are counters generated on the local device and counters reported from the remote device. So long as a device sees the correct values of its counters being reported by the remote device the router knows that the link is working. This effectively tests two way or round trip connectivity. If three consecutive keepalives are missed, or if the counter being reported does not increment properly then the router marks the interface as protocol down and will not transmit data over the link. I think that is a pretty major advantage.
The primary disadvantage is that the keepalive uses part of the link bandwidth (a very small part). If you were in a situation where you pay a per packet charge for traffic transmitted you might think twice about keepalives.
HTH
Rick
HTH
Rick
